Oxidation resistant coatings are typically applied to an engine part at varying thicknesses dependent upon the desired amount of protection. The engine part tends to incur a fatigue debit as the oxidation resistant coating increases in thickness beyond 1 mil. Such fatigue debit lessens the useful service life of engine parts that require such oxidation resistant coatings. Generally, engine parts composed of thin walled honeycomb materials, e.g., 2-5 mils thickness, are completely consumed in a typical aluminide coating process. Essentially, the material becomes a sheet of coating rather than a sheet of material having a coating disposed thereupon. The vapor aluminide coating, by its nature, is extremely brittle and breaks easily.
The thickness of the coating is directly related to the diffusion rate of the oxidation resistant coating material within the CVD chamber. Certain factors influence the diffusion rate of the oxidation resistant coating material, which impact not only the resultant coating but the article's structure and integrity as well. For instance, the application time, operating temperature and halide activator activity influence the resultant coating. Current chemical vapor deposition (CVD) processes operate at a temperature range of 1875° F. (1024° C.) to 2120° F. (1160° C.) when applying, for example, vapor aluminide coatings. The application time coincides with the hold time for the substrate, or article, being coated. At the aforementioned temperatures, the application time is approximately 30 minutes to 60 minutes. Under this time frame, the substrate develops both hot and cold zones rather than uniformly developing a hot zone throughout the substrate. For example, a hot zone may be at the optimum CVD deposition temperature throughout a majority of the application time whereas a cold zone may only attain and maintain the optimum CVD deposition temperature for a fraction of the application time. Under these conditions, the diffusion rate of the aluminum varies and subsequently deposits unevenly upon the hot zones and cold zones. The resultant coating exhibits overly thick areas and sparingly thin areas with respect to the desired coating thickness. This unacceptable non-uniform coating also contributes to inducing fatigue debit to the part.
Therefore, there exists a need for a process for applying oxidation resistant coatings to engine parts without inducing a fatigue debit to the part.